Light emitting semiconductor devices play an important role in today's lighting systems. Applications for light emitting semiconductor devices, such as light emitting diodes (LEDs) include general illumination, automotive and consumer applications. Today's technologies provide a wall-plug power efficiency of about 15%-20%, which is projected to increase up to 30% and more. Cold cathode fluorescent lamps (CCFL) being generally used in liquid crystal display (LCD) backlighting applications for notebooks, monitors, or television provide a power efficiency of about 15%. A power efficiency of about 30% pushes light emitting diodes on the same level as high frequency tubular lamps (HF-TL) being used for general illumination applications (e.g. home, office, factory, etc.).
FIG. 1 shows a simplified block diagram of a driving configuration for light emitting diodes according to the prior art. The power supply PS provides a supply voltage on a power line Vbus being coupled to a string of light emitting diodes LEDstr. The current ILED through the string of light emitting diodes LEDstr is determined by the LED driver LEDdr. The current ILED may be delivered by a linear or a switch mode regulated driver. For switch mode regulated drivers, the driver configuration can be of any kind, such as inductive buck, boost, buck-boost, and capacitive up, down, up-down topologies, or the like. A string of LEDs having one LED driver LEDdr, but multiple parallel LED strings LEDstr including LED driver LEDdr may also be used.
FIG. 2 shows a simplified block diagram of a configuration according to the prior art having two strings of LEDs LEDstr and a driver LEDdr for each of the strings. Generally, for the configurations shown in FIG. 1 and FIG. 2, the current through the strings of LEDs is the critical parameter as this current ILED determines luminance, color, brightness etc. of the light emitting diodes. The drivers LEDdr of FIG. 1 and FIG. 2 may provide direct connections to the power supply block, which are omitted for simplicity of the figures.
FIG. 3 shows simplified representations of the basic driver topologies for driving light emitting semiconductor devices according to the prior art. FIG. 3 (a) shows the linear driver configuration having a power supply PS that provides the supply voltage Vbus to the string of LEDs LEDstr and a current source CS, which determines the current ILED through the string of LEDs. FIG. 3 (b) shows a simplified schematic of the switch mode buck configuration of a driver. The switch SW is controlled to switch the current through the inductor L and the string of LEDs LEDstr such that an average current of ILED through the string of LEDs LEDstr is provided. The capacitor C functions as a buffer capacitor, and the diode D allows a current to circle through the string of LEDs LEDstr, the inductor L, and the diode D, if the switch SW is turned off. FIG. 3 (c) shows a switch mode boost configuration of a driver circuit. Accordingly, the switch SW provides a current path from power supply PS through inductor L to ground. If the switch SW is turned off, the current of inductor L continues via diode D and LED string LEDstr. FIG. 3 (d) shows a buck-boost switch mode buck-boost configuration. Accordingly, a current path is provided through the inductor L and the switch SW, if the switch is turned on. Once the switch SW is turned off, the current circles via diode D and LED string LEDstr and is driven by inductor L. Generally, a capacitor in parallel to the LED strings LEDstr can always be present to filter the LED current. Usually, the capacitor is used, when the current (typically from the coil) is not continuously flowing in the LED string LEDstr. This depends on the used driver topologies (e.g. boost and buck-boost driver topologies). The switches SW can be of n and p type. It is most convenient to use n type switches in the configurations shown in FIG. 3.
FIG. 4 shows a simplified block diagram of an electronic system according to the prior art. In particular, a solid state lighting system is depicted. The system can for example be implemented as a scanning backlight system with eight chains of 64 LEDs each. For illustrative purposes, only three chains of LEDs LEDstr are depicted in FIG. 4. An inductor L and a switch T are arranged in series with each chain of LEDs. Fly-back diodes D are coupled in parallel to the string of LEDs LEDstr. A typical LED string voltage is e.g. ranging from 173V to 237V, denoted in the following as 173V-237V. The breakdown voltage of the switches T and the fly-back diodes D should be at least the driving voltage Vbus1. If the driving voltage Vbus1=300V, then the voltage across the inductor L is either 173V-237V or 127V-63V in dependence on the switch condition of the transistors. It should be noted that the transistor, the inductor and the fly-back diode can be represented by a three terminal converter block. In the arrangement according to FIG. 4, the required breakdown voltage of the transistors T and the fly-back diodes will be 300V.
In other words, a typical architecture of circuits for driving one or more light emitting diodes includes a supply voltage applied across a string of LEDs coupled in series, and a current source or sink coupled to one side determining the current flowing through the string. The voltage drop across the string of LEDs and the voltage drop across the current source add up to the total supply voltage. Accordingly, if the voltage across the LEDs varies due to variations of the forward voltages of each LED which may be a consequence of temperature, aging or production spread, the voltage across the current source, (i.e. the driving means) may increase or decrease accordingly. If the voltage across the driving means is greater than necessary, a substantial loss of power occurs which is turned into heat. A second undesired effect of high voltages in the current sources or sinks resides in the need for components being suitable to withstand high voltages, temperatures or the like, which are a consequence of improperly adjusted voltages across the components.